For example, Laid Open German Patent document DE 100 00 305 A1 discusses the “Controller Area Network” (CAN), as well as an extension of the CAN known as the “Time Triggered CAN” (TTCAN). The media access control method used in the CAN is based on a bit-by-bit arbitration. During the bit-by-bit arbitration, a plurality of user stations may transmit data via the channel of the bus system simultaneously, without the data transmission thereby being disturbed. Furthermore, upon transmitting one bit via the channel, the user stations are able to ascertain the logic state (0 or 1) of the channel. If a value of the transmitted bit does not correspond to the ascertained logic state of the channel, the user station then terminates the access to the channel. In the case of the CAN, the bit-by-bit arbitration is usually carried out in an arbitration field within a data frame to be transmitted via the channel. After a user station has sent the arbitration field completely to the channel, it knows that it has exclusive access to the channel. Thus, the end of the transmission of the arbitration field corresponds to a beginning of an enable interval, within which the user station may use the channel exclusively. According to the protocol specification of the CAN, other user stations are not allowed to access the channel, that is, transmit data to the channel, until the transmitting user station has transmitted a checksum field (CRC field) of the data frame. Thus, a final instant of the transmission of the CRC field corresponds to an end of the enable interval.
A non-destructive transmission of the data frame via the channel is achieved owing to the bit-by-bit arbitration. Good real-time properties of the CAN are thereby obtained, whereas media access control methods, in which the data frame transmitted by one user station may be destroyed during the transmission via the channel because of a collision with a further data frame transmitted by another station, have a markedly poorer real-time behavior, since the data transmission is delayed because of the collision and the new transmission of the data frame thereby required.
A further improvement in the real-time behavior of the CAN is achieved by the TTCAN extension. According to the protocol specification of the TTCAN, a time-window structure is defined which includes a plurality of consecutive time windows (often also known as time slots) and which repeats regularly. In this case, a specific type of message, and therefore a specific user station, may be assigned a specific time window, within which messages of this message type are allowed to be transmitted. Therefore, in the case of the TTCAN, specific time windows are provided, within which one specific station has exclusive access to the channel of a CAN domain. When working with a TTCAN, the access to the channel is coordinated at least partially according to the principle of Time Division Multiple Access (TDMA).
The protocols of the CAN and of its TTCAN extension are especially suitable for transmitting short messages under real-time conditions. However, if larger data blocks are to be transmitted via a CAN domain, the relatively low bit rate then makes itself felt in an annoying manner. In order to ensure the correct functioning of the bit-by-bit arbitration, for the transmission of one bit, it is necessary to honor a minimum duration which, in particular, is a function of the expanse of the bus system and the velocity of signal propagation on the channel. Thus, the bit rate cannot readily be increased by reducing the duration of the individual bits.
To nevertheless be able to transmit a relatively large data block, needed for the programming of a control unit, with sufficient speed via a communication interface actually provided for the connection to a CAN domain, DE 101 53 085 A1 proposes switching the communication interface for transmitting the data block temporarily to a different communication mode in which no bit-by-bit arbitration is carried out, and therefore a relatively high bit rate is possible. In so doing, though, the communication using the CAN protocols must be interrupted for a certain time. For example, if, because of an error, the operation of the bus system can no longer be initiated according to the CAN protocols, a failure of the bus system then occurs. In addition, due to the transmission of a relatively large data block, the subsequent transmissions to be carried out according to the protocols of the CAN are delayed considerably, so that the real-time properties of the CAN are impaired. A use of this method not only for programming the control unit at the end of a manufacturing process of a motor vehicle or of the control unit, but also during the operation of the motor vehicle, is therefore not practical.